Published patent application DE3527942A1 discloses a temperature sensing device for measuring the core body temperature of a person or an animal. One side of the sensing device has to be positioned against the skin of body of the person or the animal. Inside the device, in a dimension perpendicular to the contact area between the sensor and the skin of the body and in the direction away from the contact area, in the order mentioned, the temperature sensor comprises a first temperature sensor, a thermal insulator, a second temperature sensor and a heating element.
If the temperature sensing device is positioned against the skin of the body the first temperature sensor measures the skin temperature. Assuming a core body temperature higher than the ambient temperature, there will be a decreasing temperature gradient in the sensing device in the direction away from the contact area between the temperature sensing device and the skin: the further a specific position inside the temperature sensing device is away from the contact area, the lower the temperature at the specific position will be. Especially, the thermal insulator creates a significant step in this gradient. Consequently, the second temperature sensor will measure a lower temperature than the first temperature sensor. As a result of the decreasing temperature gradient a heat flux will be present through the temperature sensor in the direction away from the contact area.
Note that there will be a temperature gradient in the body of the person of the animal as well. The core of the body has a higher temperature than the skin. A heat flux is present from the core of the body towards the skin. Positioning the temperature sensing device against the skin of the body influences the temperature of the skin below the sensing device. The part of the skin below the sensing device will become warmer because of the local insulating effect of the temperature sensing device. As a consequence the heat flux in the body will decrease. There is still a heat flux through the temperature sensing device as the result of the temperature gradient in the device itself. The temperature sensing device will not act as a perfect insulator and as a consequence the skin temperature will still be lower than the core body temperature.
The measured temperature difference between the first temperature sensor and the second temperature sensor is a measure for the heat flux from the contact area toward the top of the temperature sensing device. This measured temperature difference is used to control the heating element. If the top of the sensor is heated, the temperature gradient inside the sensing device, created by the thermal insulation, partly disappears. As a consequence, the heat flux from the contact area between the temperature sensing device and the skin of the body towards the top of the sensing device reduces. As a consequence the skin of the body becomes warmer and closer to the core body temperature. The heating element will be heated until the measured temperature difference between the first temperature sensor and the second temperature sensor will become very small and substantially equal to zero.
If the temperature difference measured between the first and the second temperature sensor is zero, the heat flux inside the temperature sensing device will be zero as well. If there is no heat flux through the temperature sensing device, the heat flux from the body to the temperature sensing device will be close to zero as well. If there is no heat flux between the body and the temperature sensing device, it may be assumed that the temperature sensing device has the same temperature as the core of the body.
However, it is known that the temperature sensing device of the cited art is not accurate enough. Especially in a clinical setting it is important to sense the core body temperature accurately.